Magnet wire is produced by coating conductive lengths of wire with a suitable insulator in liquid form and then feeding the coated wire through a curing oven on a continuous basis to dry the enamel. The wire is usually copper or aluminum and the insulator is typically enamel.
Traditionally, curing ovens have provided only elevated temperatures for proper curing of the enamel insulator; however, since such coatings give off volatile fumes during evaporation, the need soon became apparent for environmental control particularly with ovens having high production capacities.
There have been basically two distinct approaches to the problem of handling the evaporated solvents, which typically comprise hydrocarbons, from coatings during drying and curing. One approach has been to circulate air containing the unoxidized or partially oxidized vapors from the liquid coating through a burner, catalyst, or combination thereof to effect oxidation of the vapors. The use of a suitable catalyst, however, is expensive not only in terms of the initial expense in constructing the curing oven but also in terms of the periodic maintenance expense during shutdown of the oven to service the catalyst and remove accumulated deposits therefrom. On the other hand, the use of a burner to thermal oxidize the vapors creates other difficulties in maintaining proper temperature control within the oven due to wide temperature difference between the thermal oxidizer and the oven work chamber. Both approaches have their particular drawbacks and none of the ovens available heretofore has been completely satisfactory.
In addition, it will be appreciated that various parameters are involved including type and size of the wire, type and thickness of the coating material, rate of wire feed through the oven, etc., any one of which, if varied, could change the system balance sufficiently to require resetting, which is a manual process that can be time consuming and therefore expensive in terms of lost production. The set-up of a wire enameling oven for proper curing of the enamel and environmental control has been more of an art than a science. Different temperatures are required in zones of the work chamber to achieve evaporation then curing.
U.S. Pat. No. 3,106,386 to Harris, which is assigned to Acrometal Products, Inc., the assignee hereof, discloses a curing oven incorporating both a catalyst and a burner to effect oxidation. Heated air from the work chamber is subjected to the flame of a burner which partially oxidizes the fumes released from the enamel and carried by the air to a combustion chamber. The heated air, still containing some unoxidized fumes, is then passed through a catalyst for oxidation of the remaining fumes.
U.S. Pat. No. 3,810,736 to Dumas, which is also assigned to the assignee hereof, is an example of a curing oven which employs a burner alone to effect substantially complete thermal oxidation of the coating vapors. This oven comprises a casing with partitions therein dividing the interior into an elongated work chamber for receiving the coated wire, combustor inlet and outlet chambers interconnected by a combustor tube having a burner at the inlet end thereof, and gas passageways. A blower in the combustor outlet chamber moves air and heated gases through some passageways to opposite ends of the work chamber for passage through the curing and evaporating zones to pick up fumes evaporating from the enamel which are then carried back to the combustion inlet chamber for oxidation by the burner. In this oven, the heated air and gases from the combustor outlet chamber are mixed directly in the work chamber with the air inspirated through the open ends of the work chamber. Temperature control is achieved by setting manual dampers and by motorized volume control in order to throttle the amount of superheated air from the combustor outlet chamber with the "cold" outside air. Under conditions of reduced temperature requirements in the work chamber, proper temperature control and distribution becomes progressively more difficult to achieve due to the relatively small proportionate quantity of superheated air and combustion gases to be mixed with "cold" outside air. Moreover, any changes in the wire and coating parameters require manual readjustment of some dampers to rebalance the system. By reason of its control arrangement, the air flow in this oven could thus be characterized as constant temperature/variable volume.
A need has been developed for an improved wire enameling oven and control system therefor which is adapted to provide better temperature control and temperature distribution across the work chamber, and which is adapted to provide greater operating economy.